In recent years, leading political actors and institutions have suggested further differentiation as a plausible scenario for the EU’s future, in fields such as economic governance, social Europe, migration, tax harmonisation, and defence. One central question is to what extent such scenarios require revision of the current EU Treaties and which ones can, on the contrary, be implemented under the current Treaty text. As the chances for Treaty revision are very remote, the latter option is the more promising. Under the current rules, the various forms of differentiated integration offer different costs and benefits due, in part, to the legal conditions and constraints applying to them. For example, enhanced cooperation (wherein a group of member states can ‘use’ the EU institutions) does not allow for self-selection of participating countries and can only be undertaken for specific projects and as a ‘last resort’. By contrast, separate agreements concluded under international law between ‘willing and able’ member states do allow for self-selection of the participants, but are less effective tools as they cannot use the legal instruments of EU law. This paper presents and weighs the advantages and costs of each of the forms of differentiated integration, in relation to the various policy areas in which they might be experimented in the coming years. It does so from a legal perspective. This project received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement number 822304. The content of this document represents only the views of the InDivEU consortium and is its sole responsibility. The European Commission does not accept any responsibility for use that may be made of the information it contains.